Added TLS1.2 support to client and server.

Added MqttClientSslChannel and MqttServerSslChannel which both utilize
an SSL stream.  Added options to both MqttCLientOptions and
MqttServerOptions.  I was unable to update the code in
MQTTnet.UniversalWindows due to my lack of Windows 10.

MQTTnet.Core/Server/MqttServerOptions.cs

@@ -13,5 +13,12 @@ namespace MQTTnet.Core.Server
         public TimeSpan DefaultCommunicationTimeout { get; set; } = TimeSpan.FromSeconds(10);
 
         public Func<MqttConnectPacket, MqttConnectReturnCode> ConnectionValidator { get; set; }
+
+        public bool UseSSL = false;
+
+        /// <summary>
+        /// The path to the X509 SSL certificate.
+        /// </summary>
+        public string CertificatePath = string.Empty;
     }
 }